A cascade silencer made by connecting sub chamber configurations is studied by Yu and Cheng (2015). Three types of expansion chambers as shown in figure 14 are designed and analysed individually for noise reduction performance.
Figure 14.Three types (a), (b), (c) and a 3D view of type (b) (Yu and Cheng, 2015, p 1).
As shown in figure 14, the chamber width w, single side extension lengthr and double side extension length q are considered as geometric variables. When increasing chamber width from 0.05 m to 0.1 m, it is observed that the peak transmission loss occurs in higher frequency region. By increasing the length r from 0.03 m to 0.07 m, the peak transmission loss shifts to a low frequency region due to increase in characteristic height of chamber and there is also an increase in bandwidth at transmission loss value of 20 dB. In case of increasing length of double sided extensions,q, the bandwidth at 20 dB transmission loss is reduced and there is a shift of peak transmission loss towards lower frequency region. (Yu
& Cheng 2015, p 2.) Effect of cascading all the three sub chambers added with one more chamber of type (a) on sound transmission loss is illustrated in the figure 15 (Yu, Tong, Pan
& Cheng 2015, p. 66).
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Figure 15. Performance of individual and cascaded chambers (Yu, Tong, Pan & Cheng 2015, p. 66).
Figure 15 shows the transmission loss for four individual sub-chambers and a cascaded chamber in the order 1, 2, 3, 4: Chamber 1 is of type (c) with q = 0.07 m, chamber 2 is of type (b) with r = 0.06 m, chamber 3 and 4 are of type (a) with w = 0.08 m and 0.125 m respectively. It can be noted that the performance of cascaded chamber is remarkably better than that of individual chambers. By considering a target transmission loss of 20 dB, an efficient blocking of sound from 280 Hz to 1380 Hz is achieved and by increasing the target level to 40 dB, a complete blocking is achieved from 500 Hz to 1350 Hz as a result of cascading the sub-chambers. (Yu, Tong, Pan & Cheng 2015, p. 66.)
Multiple partitioning of sound passage is also applicable for the sound flow through orifice.
Noise transmission through a duct with an orifice is studied by Qian et al. (2015). The orifice with single flow passage without any obstacle and orifice with multiple flow passage guided by a thick perforated plate are compared. The total area of cross section of multiple flow passage is maintained equal to that of single flow passage. The effect of splitting the flow passage on sound transmission loss is examined as shown in figure 16. (Qian et al. 2015, p.
90.)
Figure 16.Sound transmission via orifice and perforated plate (Qian et al. 2015, p. 90).
It can be noted from the figure 16, that the frequency bandwidth, f in which there is a continuous transmission loss (TL), is increased from 2750 Hz to 3100 Hz by replacing the single flow orifice passage with thick perforated plates guiding multiple flow passages. It can be understood from the above two examples, that by creating partitions along or across the sound flow passage, a positive effect on noise reduction could be achieved. There is relatively a wider scope for design development of duct acoustic silencers and mufflers to increase its noise reducing performance and compactness. This may give a simpler solution for noise reduction compared to modifying the properties of duct or fluid. However, there are only limited number of literature dealing with studies of the effect of geometrical variations of structures on noise reduction performance. (Qian et al. 2015, p. 87.) As the manufacturing freedom of complex designs has been increased by AM technologies, a better solution for noise reduction could be achieved by utilizing the manufacturing capability of AM.
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4 ADDITIVE MANUFACTURING
Additive manufacturing is a manufacturing technology that follows principle of adding material mostly in a layer by layer fashion to build a component. Many variants of equipment had been developed in additive manufacturing family in the last three decades. The capabilities and limitations of equipment variants differ from each other in terms of material, accuracy, geometric complexity, mechanical properties and economic consideration.
Powder bed fusion (PBF) (ASTM F2792-12a 2013, p. 2) is one of the AM processes in which almost fully dense material can be built up layer after layer to consolidate parts of definite geometry as shown in figure 17 (Scotti et al. 2016, p. 476).
Figure 17.Powder bed fusion process (Scotti et al. 2016, p. 476).
As shown in figure 17, a 3D model developed by computer aided design (CAD) is sliced into 2D layers and used as input geometry for fabrication. A numerically controlled laser beam guided by scanner optics is used to melt each layer. The powder bed platform lowers to a distance equal to exact layer thickness and fresh powder is evenly spread over the platform after melting every layer. A main advantage of PBF process lies in economic manufacturing of parts with higher level of complexity. This process is relatively slow compared to other AM processes. Fused deposition modelling (FDM) (ASTM F2792-12a 2013, p.2) is one more commonly used AM process especially for acrylonitrile butadiene styrene (ABS) and poly-lactic acid (PLA) polymers in which the material is added through
nozzle layer by layer with a constant pressure as shown in figure 18 (Novakova-Marcincinova 2012, p. 36).
Figure 18.Fused deposition modelling (Novakova-Marcincinova 2012, p. 36).
As shown in figure 18, the material filaments are fused by heated nozzles and deposited layer by layer to build the main part and support structure. The accuracy of part depends on the radius of the nozzle opening. The layer thickness typically ranges from 0.18 mm to 0.35 mm.
(Kannan & Senthilkumaran 2014, p. 1048) Higher layer thickness imposes limitations while building the parts with higher geometrical complexity and thinner structures. Thus PBF process is applicable for manufacturing a wider range of complex designs which are useful for building complex cavity structures for noise reduction. In case of building thin wall structures in PBF process, the quality of part depends on the form, dimensional accuracy and porosity. (Abele et al. 2015b, p. 119; Gu & Shen 2008, p. 1884) It is essential to consider certain procedures in design and manufacturing for PBF process in order to avoid manufacturing failure and to ensure part quality.